1. Field of the Invention
The invention relates to linear arrays of sonar hydrophones adapted for towing behind ships and submarines and in particular to the reduction of the influence of variational noise on low frequency performance.
2. Discussion of Prior Art
The acoustic part of a towed array of the type referred to herein comprises a number of sections, or modules, of long oil-filled hose containing hydrophones. Each module contains within the oil cavity, high modulus, longitudinal strength members, hereinafter referred to as internal strength members, whose main purpose is to support the tow load. In addition, there may exist longitudinal reinforcement embedded in the hose wall to prevent the latter from stretching. The two ends of each module are terminated in solid couplings which serve to join the sections together and to transfer the tow load from the hose wall to the internal strength members. It follows from this construction that if one of the couplings is vibrating longitudinally, whatever the source, then all the other couplings will be vibrating in some measure, since they are linked together by the high modulus internal strength members.
The primary sources of vibration are strumming of the tow cable, and snatching of the tow cable at the tow point when the towing vessel is in a heavy sea. Current methods for reducing the vibration are either to fair the cable or to insert vibration isolation modules between the cable and the array. These methods are not always totally efficient, and even when they are, there still exists a threshold level of vibration generated in the array itself by the action of the turbulent boundary layer.
It is known that the internal strength members are important transmitters of low frequency, longitudinal vibration. This sets the couplings into vibration which act as discrete sources of bulge waves (sometimes known as breathing waves) and longitudinal hose-wall elastic waves, each of which generate pressures inside the hose. Since these waves have long wavelengths at low frequencies they can propagate to the centre of a module with little attenuation from hose-wall damping. Reflections from adjacent couplings, partial reflections from internal components, and standing wave effects may also be important.
It is evident that the resultant self-noise field due to low frequency vibration is not spatially homogeneous. In other words, the self-noise levels on single channels vary with position in the module. As a consequence, the familiar concept of wavenumber space that is so useful for interpreting the characteristics of the homogeneous pressure response at high frequencies, is no longer very useful at low frequencies. A related problem is that the spatial non-homogeneities render difficult any simple estimation of the array gain against self-noise to be applied to single channel levels.